Spinal glycine receptor alpha 1 coordinates startle behavior through a cell-type specific mechanism.

The spinal cord critically regulates startle responses through local excitatory and inhibitory circuits. The glycine receptor alpha 1 subunit (GlyRα1) mediates the primary inhibitory neurotransmission in the spinal cord, however, the contribution of spinal glycinergic inhibition to startle regulation remains poorly understood. Here, we show that GlyRα1 controls startle behavior in a cell-type- and region-specific manner. GlyRα1 deletion in ChAT-positive spinal neurons markedly enhances startle responses and c-Fos activation in spinal motor neurons as well as excitatory interneurons. In contrast, GlyRα1 deletion in inhibitory interneurons suppresses startle behavior while selectively increasing spinal c-Fos activation in inhibitory but not excitatory interneurons. Consistent with these findings, intraspinal AAV-Cre induced deletion of GlyRα1 without cell-type specificity increases startle responses, whereas deletion from spinal CamK2α-positive cells, which predominantly overlap with glycinergic inhibitory interneurons, attenuates startle reflexes. By contrast, deletion of GlyRα1 from brainstem RtTg CamK2α-positive cells, which are predominantly glutamatergic, enhances startle responses. GlyRα1 deficiency in glycinergic neurons reduces the amplitude of puff-applied glycine-elicited currents without affecting synaptic inhibitory and excitatory neurotransmission in the spinal cord. Together, these findings identify spinal GlyRα1 subunits as a cell-type specific regulator of startle behavior and reveal opposing contributions of GlyRα1 signaling in spinal inhibitory interneurons and motor/excitatory output pathways.